This invention relates to a method and apparatus for bonding plastic materials, and more particularly relates to heat sealing the interface of two plastic tubular members to form a self-supporting assembly.
It is necessary to permanently connect two plastic members for a variety of applications. The connection is generally accomplished by heating the interface of the plastic members to a bonding temperature and applying an appropriate pressure with the heat at the bonding temperature. However, to effectively and firmly seal two plastic members it is necessary to provide a bond that is smooth and wrinkle-free. This is difficult to accomplish primarily because of problems in controlling the proper bonding temperature, pressure and heat location for sealing. These problems are accentuated when plastic materials having various thicknesses and different bonding characteristics are being sealed.
In this regard, it is known to use various sources of heating energy such as induction, dielectric, radiant and conductive sources. For example, Taylor et al, U.S. Pat. No. 4,119,825 shows an induction heating apparatus for a dieless drawing operation to form metals. The apparatus includes a flexible induction coil made from a flexible metallic strip contained within an elastomeric sleeve that surrounds the workpiece being heated and substantially conforms to the surface of the workpiece. The coil however is not seamless and thus may cause a discontinuous seal if used with plastic members. Also, the coil does not collapse about the workpiece or supply any bonding pressure as would be necessary with plastic members.
Various methods have been utilized to heat seal two plastic members. One such method is particularly adapted for sealing tubular members, and employs a discontinuous metal band or bands positioned about and in contact with the tubular members to be sealed. The metal band or bands are then heated, and a seal is formed along the interface between the two plastic members. However, this method may cause bunching of the plastic being sealed as well as undesirable voids or wrinkles in the bond causing a less effective seal.
A particularly useful method of generating heat for joining two plastic members uses discrete susceptor particles within the surfaces to be joined and applying a high frequency magnetic field. See for example, Heller, Jr. et al, U.S. Pat. No. 4,035,547; Heller, Jr. et al, U.S. Pat. No. 3,574,031; and Leatherman et al, U.S. Pat. No. 3,528,867. In this prior art, the susceptor particles may be located either directly upon or within the surfaces to be joined or supported within an intermediate base interposed between the surfaces to be joined. When subjected to the high frequency magnetic field of an induction heating coil, the susceptor particles generate heat resulting in a fusion bond along the interface of the plastic members. In this and similar methods a pressure is applied across the members during the bond to create a firm and continuous interface bonding.